In large-capacity power conversion devices, output of a power converter is high voltage and large current, and therefore many of the large-capacity power conversion devices are configured such that a plurality of converters are multiplexed in series or in parallel. The multiplexing of the converters is not only for the purpose of increasing the capacity of the power converter. By combining outputs of the converters, harmonics contained in the output voltage waveform of the power converter are reduced, and as a result, harmonic current flowing out to a grid can be reduced.
There are various methods for multiplexing the converters, such as reactor multiplexing, transformer multiplexing, and direct multiplexing. The transformer multiplexing has an advantage that DC currents of the converters can be made to be the same current because the AC side is insulated by the transformer. However, there is a disadvantage that, if the output voltage is high voltage, the configuration of the multiplexed transformer is complicated, and the cost of the transformer increases.
Accordingly, as a power conversion device that is suited for high-voltage usage and that does not need a multiplexed transformer, a multilevel converter is proposed in which outputs of a plurality of converters are connected in cascade, and one of such multilevel converters is a modular multilevel converter (hereinafter, referred to as MMC).
The MMC is formed by an arm in which a plurality of unit converters (hereinafter, referred to as converter cells) which are called cells are connected in cascade. Each converter cell includes a plurality of semiconductor switching elements and a DC capacitor. Then, through ON/OFF switching of the semiconductor switching elements, both-end voltage of the DC capacitor and zero voltage are outputted.
In the case of three-phase MMC, the aforementioned arm is formed individually for each phase. These phase arms are connected in parallel to each other, and connection terminals at both ends connected in parallel are used as DC terminals. Each phase arm is composed of a positive arm and a negative arm each having a half of the total converter cells connected in cascade. The connection point between the positive arm and the negative arm serves as an AC-side input/output terminal.
Since outputs of each converter cell in the MMC are connected to both of the AC end and the DC end of the MMC, each converter cell has a characteristic of outputting both of DC current and AC current. That is, current flowing through each arm contains an AC component and a DC component. Therefore, in the MMC, the plurality of current components are controlled.
In addition, since the MMC is connected to both of the AC end and the DC end, it is necessary to cope with short-circuit, grid disturbance, and the like that can occur at each terminal. In particular, in the case where short-circuit or the like occurs at the DC end, power transmission is stopped until the short-circuit is eliminated. Therefore, it is necessary to swiftly eliminate the short-circuit and restart rated power transmission. If short-circuit occurs, the voltage of the DC end becomes zero, and therefore it is necessary to raise the DC output voltage of the power conversion device to the rated value in order to start power transmission after the short-circuit is eliminated. As a method for restarting the power conversion device without influencing the grid, a power conversion device restart method is disclosed in which DC voltage is gradually raised in a state of being interconnected with the AC grid, whereby overvoltage of DC lines is suppressed and the DC voltage is stably restored as follows.
Each arm of a power converter is formed by two types of unit converters, i.e., full-bridge-type unit converter and bidirectional-chopper-type unit converter. A command value distributing unit distributes an AC voltage command value and a DC voltage command value to an output voltage command value for a bidirectional chopper group and an output voltage command value for a full-bridge group. A gate pulse generation unit generates gate signals to be given to the respective full-bridge-type unit converters and gate signals to be given to the respective bidirectional-chopper-type unit converters so that the voltage command values and the actual voltages coincide with each other as much as possible.
A DC failure detection unit changes a DC failure detection signal from 0 to 1 after a certain time period has elapsed since detection of DC failure. At this time, the DC failure detection unit changes the DC failure detection signal from 0 to 1 in a ramp function shape with a certain slope. The DC failure detection signal is given to the above command distributing unit. Along with change of the DC failure detection signal, the command value distributing unit returns the output voltage command value for the bidirectional chopper group and the output voltage command value for the full-bridge group, to the waveforms similar to those at the time of occurrence of DC failure (see, for example, Patent Document 1).